MINERALOGY. 540 



iodide cbauges suddenly to the red hexagoual, and be argues that true 

 bexagoual structure may result from molecular twinningof orthorhombic, 

 molecules. 



Other investigators have worked on the same subject, of the change 

 produced by elevation of temi)erature. Doelter {Jahrh. Min., 1884, ii, 

 217) Las experiniented upon vesuvianite, apatite, and tourmaline. W. 

 Klein, of Bonn, has published an extended paper {Zeitsch. Kryst, ix, 

 38), in which he gives the, results of similar experiments with apatite, 

 quartz, apophyllite, zircon, iolite, heulandite, and other minerals. 

 Forstner {Th., p. 333) gives the results of a very interesting series of 

 experiments upon the changes in optical characters brought about by 

 artificial change of physical conditions on the feldspars of Pantelleria. 

 These lead to essentially the same conclusions as the exi^eriments of 

 Klein on boracite and leucite. Among other things Forstner says, 

 with respect to microcliue-albite, that, considering the volcanic origin 

 of the feldspars, it is natural to conclude that at the time of its forma- 

 tion the conditions as to temperature, &c., were such as to cause a 

 monoclinic arrangement of the particles, and that the present (triclinic) 

 optical characters are the result of subsequent changes. 



From a theoretical standpoint, perhaps the most interesting and sug- 

 gestive contribution to this subject is a paper by Mallard {Bull. Soc. 

 Min. France, vii, pp. 349-401), in which he discusses the probable molec- 

 ular arrangements in the different types of crystals and the changes 

 which they undergo under changes of conditions, as of temperature, &c. 

 He calls attention to the cubic type, and the fact that the crystals of 

 other systems often approximate to itj and, moreover, that the crystal- 

 line parameters of a very large number of crystals are simple multiples 

 (1 , 2, 3) of the parameters of the cubic " r^seau." From this he concludes 

 that for all crystals, without exception, the r^seau formed by the centers 

 of gravity of the molecules is the same, and very nearly that which char- 

 acterizes the cubic " r6seau." The relatively slight differences which 

 exist between this last and the one actually existing could constitute 

 with the special molecular twinning, and the molecular volume, the crys- 

 talline individuality peculiar to each substance. This molecular twin- 

 ning, which heat and other causes can alter, is regarded as giving rise to 

 the true crystalline molecules. Keference must be made to the original 

 paper for the full discussion of the subject. 



Des Cloizeaux has continued his investigations of the optical charac- 

 ters of the feldspar species. A recent paper {Btdl. Soc. Mm., vii, 249) 

 is devoted to oligoclase and andesine, and covers nearly 100 pages, 

 giving a large number of determinations of the position of the axes of 

 elasticity, measurements of the axial angles, and so on. His observa- 

 tions go to show that, in the albite-anorthite series of plagioclase, 

 the species or subspecies andesine deserves a more definite place than 

 many mineralogists have of late years been inclined to give to it. It 

 may be added here that Gylling has recently described {Jahrh. Mw., 



